Because of the unfavorable polarization selection rules for intersubband transitions, a vertical cavity surface emitting laser [1]}, which is a usual choice for small volume and low dissipation devices, is not a geometry that can be easily implemented for intersubband quantum cascade lasers (QCLs) [2]}. A promising alternative structure, as shown in Fig. REF (a), is a short, tightly-confining narrow-ridge active region, in our case a QCL processed into a buried heterostructure with ridges down to 1 \(\mu \) m width. In this configuration, keeping the optical losses to an acceptable value requires facet coatings ideally approaching > 98% modal reflectivity.
